The Necker Island Nature Tour is great. Especially if you like animals. You will see lemurs, flamingos, other exotic birds and giant tortoises.

There are no dinghy lines or moorings at Prickly Pear for snorkeling. I attached a map of the best snorkel spots in the North Sound and surrounding areas. My personal favorite is Eustatia Reef on a calm day. If it is rough, you can go to Cactus Reef at the northwest side of Prickly Pear just off the main channel into the North Sound. There is a tiny beach there where you can beach your dinghy and snorkel right from there.

Mountain Point is great snorkeling, I also enjoy Fallen Jerusalem. Savannah Bay doesn't have too much going on unfortunately.

The south side of Great Dog tends to be one of the most protected snorkel spots in my opinion when the swell is up. But honestly I enjoy anywhere at The Dogs. Just condition dependent.